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Abstract

Relatively long (30 µm) high quality ZnO nanowires (NWs) were grown by the vapor-liquid-solid
(VLS) technique. Schottky diodes of single NW were fabricated by putting single ZnO
NW across Au and Pt electrodes. A device with ohmic contacts at both the sides was
also fabricated for comparison. The current-voltage (I-V) measurements for the Schottky diode show clear rectifying behavior and no reverse
breakdown was seen down to -5 V. High current was observed in the forward bias and
the device was found to be stable up to 12 V applied bias. The Schottky barrier device
shows more sensitivity, lower dark current, and much faster switching under pulsed
UV illumination. Desorption and re-adsorption of much smaller number of oxygen ions
at the Schottky junction effectively alters the barrier height resulting in a faster
response even for very long NWs. The NW was treated with oxygen plasma to improve
the switching. The photodetector shows high stability, reversibility, and sensitivity
to UV light. The results imply that single ZnO NW Schottky diode is a promising candidate
for fabricating UV photodetectors.

Introduction

Zinc oxide (ZnO) is a unique material with semiconducting and piezoelectric dual properties.
It is turning out to be a very important material due to its wide variety of potential
applications in everyday life like sunscreens, miniaturized lasers, light sources,
sensors, piezoelectric elements for power nano-generators, transparent electrodes
[1] etc. ZnO has many advantages over other wide bangap semiconductors like direct band
gap of 3.37 eV, large excitons binding energy of 60 meV, high thermal/chemical stabilities,
and the option of wet chemical etching etc. [1,2]. This has led to the demonstration of ZnO as an alternative material to the nitride
semiconductors.

ZnO has a rich family of nanostructures such as nanowires, nano belts, nano particles,
nano tips, and nanotubes [1,3]. ZnO nanowires (NWs) have attracted significant attention due to their large surface
area, good crystal quality, and unique photonic properties. One-dimensional nanocrystal,
for instance, a NW can serve as a sample for studying the low-dimensional phenomena
and is potentially a building block for the complex nanodevices.

P-type doping of ZnO is still a problem that diminishes the prospects of a ZnO p-n homojunction device [4]. On the other hand, ZnO is naturally n-doped and does not need external dopants. A Schottky diode seems to be a very feasible
device from ZnO. A Schottky barrier diode exhibits faster switching and lower turn-on
voltages as compared to a p-n junction diode and there is some optical loss in the p-region of a p-n diode. That makes it a very useful for electronic and optoelectronic application.

In the past few years, there has been an increased interest in one-dimensional NW
based UV sensors and these demonstrated potential applications as next-generation
of UV sensors [5-8]. However, there are relatively much less reports on comparative study of photosensitivity
dependence on the type of metal semiconductor junction. This article reports our UV
response measurements of a Schottky-junction diode made of a single ZnO NW in comparison
with a ZnO NW with ohmic contacts on both the sides. Very long NWs (approx. 30 µm)
were used in this study that show very fast response on full length device (due to
the reduced dimensionality of the active area at the Schottky junction) and potentially
allows fabrication of several diodes on a single NW.

Experimental

Relatively long (30 to 40 µm) crystalline ZnO NWs with a lateral diameter of approx.
100 nm were grown by high temperature (approx. 900°C) vapor-liquid-solid (VLS) technique.
For the growth of ZnO nanowires, a thin film of pure Au (99.9%) was used as a catalyst
and was deposited on the Si substrate in a high vacuum metallization chamber. The
thin gold film melts into small gold droplets at elevated temperature, which act as
growth sites for ZnO nanowires. The source material was prepared by mixing graphite
(99.9%) with ZnO (99.9%) powder with ratio of 1:1. The source material was placed
into a ceramic boat and the substrate was placed 3 to 4 cm away in the downstream
and the growth face was downward to the source material. Zn, CO, and CO2 gases are produced from the reaction of ZnO and graphite powder at 900°C. Zn atoms
adsorb on the Au droplet surface due to higher sticking coefficient of Zn on liquid
versus solid. CO/CO2 molecules are transported to the liquid-solid interface and bulk diffusion of Zn takes
place through Au droplet [9]. Zn islands oxidize to ZnO due to the presence of CO/CO2 mixture. The argon gas was used as a carrier gas with flow of 50 to 80 sccm (standard
cubic centimeters per minute). The growth temperature was approx. 900°C. The growth
time was about 40 min. The schematic of the process is shown in Figure 1.

The samples were annealed at 600°C in ambient argon to improve the crystal quality
and minimize the defects. High-resolution transmission electron microscopy (HRTEM)
image (Figure 2a) indicates the good monocrystalline quality structure of the ZnO NW. Lattice spacing
is approximately 0.26 nm between the two adjacent (002) lattice planes and it confirms
the <0001> growth direction [10]. Furthermore, the X-ray diffraction (XRD) pattern of the ZnO nanowires is shown in
Figure 2b. The strong (002) peak and weak (004) peak reconfirm that the ZnO nanowires preferentially
grow along the c-axis <0001> direction. Higher intensity and narrow spectral width of the (002) peak
affirms that the grown ZnO has high-purity wurtzite hexagonal phase [11].

Figure 2.Structural characterization of the ZnO nanowires. (a) HRTEM image of the edge of an as-synthesized ZnO nanowire. The spacing of 0.26
nm between adjacent lattice planes corresponds to (002) lattice planes of ZnO and
<0001> growth direction is also shown; (b) XRD spectrum of the ZnO nanowires.

NWs were then ultrasonically dispersed in ethanol and were placed onto a gold patterned
insulated SiO2 substrate. After drying out the suspension, gold was re-evaporated onto one side of
some selected ZnO NWs by lithography and lift-off process in order to form a stable
Schottky contact. Focused ion beam (FIB) is used to deposit Ga induced Pt on the contact
between the NW and the gold electrodes for eliminating the Schottky barriers and form
high quality ohmic contacts [12]. Some samples were prepared by making ohmic contacts to both the sides. The scanning
electron microscopic (SEM) images and the schematics of the device are shown in the
Figure 3.

Figure 3.Two types of devices fabricated for comparison. (a) Electric model, schematic and SEM of the fabricated Schottky diode, I-V showing
good rectifying behavior; and (b) Electric model, schematic and SEM of the device
with ohmic contact on both sides, I-V show clear ohmic behavior.

Results and discussion

In Figure 2b, the I-V characteristics show a linear behavior between the two Pt ohmic contacts at the room
temperature. This verifies that both the Pt electrodes show a good ohmic behavior.
The I-V characteristics of our ZnO NW Schottky diode shown in Figure 3a demonstrate a good rectifying behavior.

Threshold voltage was also observed to be shifted from 2 to 1.2 V as temperature rise
from 80 to 340 K. Electron transport through the Schottky barrier is described by
thermionic emission as well as by small tunneling current. The thermionic current
produces the rectifying I-V curve and dominates electron transport. The forward bias I-V characteristics in the Schottky-junction diode were analyzed using the thermionic
emission model given by [13,14];

where η is the ideality factor, k the Boltzmann constant, T the temperature, Rs is the series resistance, and ISAT is the reverse saturation current. The ideality factor values were found to be >3,
which indicates that some non-thermionic processes also contribute to the conduction
[13,15]. The barrier height is calculated from the relation:

where A is the area of the diode, φbthe Schottky barrier height (SBH) of the junction, and A* the Richardson constant, which is 32 A cm-2 K-2 for ZnO [15]. SBH was calculated to be 0.48 eV with an ideality factor of 3.1. These unusual electrical
characteristics of our single ZnO NW Schottky diode can be explained by a thermionic
field emission and an enhancement of the tunneling effects due to both the naturally
high carrier concentration of the ZnO NW itself and the nanoscale junction size of
the NW Schottky diodes [13].

Photoconductive response is a key figure of merit for a photodetector. Response of
both the ohmic and Schottky devices was measured using a 365-nm UV source at a bias
of 0.5 V (Figure 4). The real time ON/OFF switching was measured by applying a UV pulse with an intensity
of 1.5 mW/cm2. The measured photocurrent shows a rapidly rise and fall upon exposure to UV light
for the Schottky detector and the current decreases down to 35 nA, which is quite
close to the initial value under dark. Photocurrent pulse shows good stability and
reversibility. Whereas, the recovery time is much higher for the ohmic detector and
the value of the dark current is also relatively higher. Thus, the Schottky diode
shows much faster switching under pulsed UV illumination as compared to the device
with ohmic contacts on both the sides.

Figure 4.Photoresponse of a single ZnO nanowire under pulsed illumination by a 365 nm wavelength
UV light with (a) Schottky contact on one side, and (b) ohmic contacts on both sides.

Under dark condition, oxygen molecules are adsorbed on the NW surface and capture
free electrons from the n-type ZnO, making negatively charged O2 ions at the surface. This creates a low conductivity depletion layer near the NW surface:

When the UV exposure is made, electron-hole pairs are photo-generated and holes are
trapped at the surface by the oxygen ions via surface electron-hole recombination:

Unpaired electrons are left behind which add to the photocurrent [5,6]. Thus, the NWs are very suitable for obtaining higher sensitivity of the devices
due to an enhanced surface to volume ratio. Schottky barrier demonstrates hole-trapping
in the reversed bias junction that reduces the depletion region and assists tunneling
of additional electrons [16].

When the UV illumination is switched on or off, the oxygen is desorbed or readsorbed
in the interfacial region in the premises of the metal contact in Schottky diodes
and it reduces the Schottky barrier height, whereas for the device with ohmic contacts
on the both sides, it happens throughout the NW surface. This explains the better
sensitivity and faster switching of the photocurrent in the Schottky barrier devices
as compared to the device with ohmic contact on both sides. This can be useful for
carrying out single photon detection [5] as the adsorption and desorption of small number of oxygen ions at the junction area
can effectively alter the barrier height. Usually it was considered advantageous to
use short length NW for faster switching but with this Schottky barrier approach,
even the longer NWs (approx. 30 µm in our case) are equally responsive. This allows
for the possibility of processing multichannel NW devices with conventional photolithography
as on most of the previous occasions [5,13,17,18] e-beam lithography is compulsory due to the very small NW lengths.

The NW was then treated with oxygen plasma under an oxygen flow rate of 100 sccm,
chamber pressure of 150 mTorr for 1 min. Photocurrent was observed to decrease after
the oxygen plasma treatment but photocurrent rise and fall time under UV exposure
is further improved significantly as compared to the untreated ZnO NW, as shown in
Figure 5.

Figure 5.Photoresponse of a single ZnO nanowire Schottky diode under pulsed illumination after
oxygen plasma treatment.

Oxygen vacancies act as electron donors inside ZnO. Oxygen plasma treatment causes
oxygen ions to diffuse into the ZnO NW to fill the oxygen vacancies. This results
in the reduction of the total photocurrent. Whereas, surface defects and charged species
for trapping and scattering the carriers increase after the oxygen plasma treatment
thus this surface modification works in favor of faster switching.

Conclusion

In summary, Schottky diodes of very long (approx. 30 µm) single NW were fabricated
by putting single ZnO NW across Au and Pt (Ga induced) electrodes. A device with ohmic
contacts to both the sides was also fabricated for comparison. UV photoconductive
response of both the ohmic and Schottky devices was measured. The Schottky barrier
device shows more sensitivity, lower dark current, and much faster switching under
pulsed UV illumination. Desorption and re-adsorption of much smaller number of oxygen
ions at the Schottky junction effectively alter the barrier height resulting in a
faster response even for very long NWs, thus making possible the processing of the
device by conventional techniques. The oxygen plasma treatment further enhances the
switching. The photodetector show high stability, reversibility, and sensitivity to
the UV light. Thus, a complete recipe for a UV photodetector capable of fast switching
is concluded out of the present research.